Workpieces such as typical automotive engine hydraulic valve lifter bodies often have the configuration of a thin-walled cylindrical tube whose length can be from three to four times the diameter and have one end of the tube closed by an end wall. As a result of the thin wall and length of the tube relative to its diameter, prior art workers have had difficulty in devising a mechanism to fixture the lifter body with sufficient restraint of the body to hold it in desired position for grinding without at the same time elastically deforming the workpart from the fixturing forces and causing resultant increase in roundness and straightness deviation on the ground lifter body.
The lifter body has been gripped in the past by a chuck near the closed end of the lifter body. Oftentimes, this type of fixturing is not sufficient to enable the lifter body to resist grinding forces or moments which are distributed along the entire surface being ground; e.g., along the inner bore of the tube that is ground along its length. The valve lifter body typically is too weak near the open end of the tube to permit gripping by a chuck without elastically deforming the open end.
The Seidel U.S. Pat. No. 3,209,494 issued Oct. 5, 1965, illustrates a method and apparatus for grinding an annular thin wall workpart of relatively short length relative to its diameter to minimize distortion of the thin wall workpart when the grinding force is applied by allowing the annular workpart to laterally and bodily shift to off-center positions relative the longitudinal axis of the fixture. The workpart is thus not fixedly clamped in position during grinding. The fixture that permits such shifting of the position of the workpart includes a support member having a face plate against which one end of the workpart is magnetically held and having multiple fluid passages adjacent the other end to direct fluid against the workpart surface not being machined in a direction to urge the workpart to a centered position. As the workpart shifts off-center from grinding forces exerted thereon, differential fluid pressure is established around the workpart to urge the workpart to return back to the center position.
Australian Pat. No. 121,287 discloses an aircraft engine cylinder that is pressurized from the inside while the outer surface is being ground to prevent distortion of the thin wall of the cylinder from grinding forces and to provide internal cooling action. Both ends of the cylinder are supported by end plates carried on live-centers. The end plates suitably close off the open ends of the cylinder to maintain fluid pressure in the cylinder bore during grinding of the outer surface.